Thermal spraying of a piston ring

ABSTRACT

The invention relates to a piston ring coated with a coating material by a thermal spray process, exposed to heat treatment of the coating material at an elevated temperature and for a time effective to at least partially diffuse the coating material into the piston ring surface or underlying layer of coating material, and an additionally applied coating material layer subject to successive heat treatments of each coating material layer in order to lay down on the piston ring surface a plurality of layers of the same coating material.

FIELD OF THE INVENTION

The present invention relates to a piston ring and a method of applyinga coating material to a piston ring surface, said method comprising thefollowing steps, application of said coating material by a thermal sprayprocess and heat treatment of said coating material of said piston ring.

TECHNICAL BACKGROUND

Very specific demands have to be met by high temperature applicationssuch as piston rings that are intended for use in for instance marinediesel engines, particularly as concerns strength, anti-corrosiveproperties, wear resistance, ductility and material resilience.

When used in a diesel engine the piston ring is arranged to abut on theone hand against an associated piston groove, on the other against anengine cylinder-bore. Consequently, the ring should be wear-resistant,particularly at the interface towards the cylinder bore, where highfriction is generated when the engine is in operation.

The piston ring should therefore also possess an inherent tension orresilience whereby the piston ring will constantly be forced outwards,into abutment against the cylinder bore. In addition, upon eachexplosive stroke of the engine, the piston ring is urged withconsiderable force radially outwards, into abutment against the cylinderbore, with consequential increase of stress. Due to a high workingtemperature in engines and especially due to the impact of producedheat, from contact between piston rings and cylinder liner during theprocess, many materials loose some of their yield strength and showsoftening.

Today, piston rings are generally manufactured from a cast-iron blank,which substantially meets the requirements imposed on the material asregards strength and resilience but generally not, especially whenheated, on wear resistance on the surface thereof that faces thecylinder bore. Cast iron does not possess the required thermal stabilityat high temperature. A cast-iron piston ring blank therefore usually isprovided with a wear-resistant layer on the surfaces most exposed towear.

However, difficulties do arise in achieving a sufficiently strong bondbetween the material of the blank and the material of the wear layer,which causes problems, because of the risk that the material of the wearlayer be torn away from the material of the blank. When this happens,the comparatively soft material of the blank-material surface is exposedto wear in the area of contact against the cylinder bore, with resultingconsiderable shortening of the life of the piston ring.

Another issue is that the coating gradually wears away, even if the bondbetween the surfaces is comparatively strong. The wear on the pistonring progresses slowly as long as the wear layer is intact but veryrapidly, once that layer has been damaged. As a result, it may bedifficult to determine in time when a piston ring change should be made.

Thermal spraying is conveniently used to apply coatings on piston rings.Generally, one issue with using thermal spraying for applying a coatingis that the resulting coating comprises a fraction of loose particles.These loose particles are increasing the risk for “three-body-wearing”between for example the coated piston ring and cylinder liner.Three-body-wearing is often an initiator of the process of gradualwearing indicated in the above.

In operation, especially some contact areas between the piston ring andcylinder bore or liner material are exposed to high temperatures, toconsiderable temperature differences, and to the effects of a highlycorrosive environment. In order to withstand the effects of thesestress-inducing causes, the piston ring therefore also must exhibitconsiderable ductility, and thermal stability in addition to the beforementioned wear resistance. By ductility is to be understood herein themaximum possible deformation of the material before cracking begin.

It is thus desirable to achieve a coating for piston rings, which isresistant to wear, thermal chock, corrosion and oxidation. It is knownvarious methods to post-heat or sinter a substrate after a coating hasbeen applied in order to provide a strong bond between coating andsubstrate. U.S. Pat. No. 5,268,045 provides an example of such prior artcoating method, wherein a work piece to be coated is electrochemicallycleaned, thermal spray coated with a metal or metals in order to providean overlay coating and post heat treated at an elevated temperature,typically for about two hours, to diffuse said metal or metals into thesurface of the work piece.

During such a process the coating will reach its melting temperature andthere is a risk that also the underlying substrate is affected in such away that stress is induced in the substrate. This is especially an issuefor piston ring blanks. Other issues related to prior art techniqueswill be described more in detail in the document. Hence it is desirableto find a method for applying a coating on a piston ring with minimziedinduced stress in the piston ring blank and also reducing the risk ofloose particles within the coating. At present no method is known ofapplying a coating for piston rings to overcome the above issues.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method of applying acoating, particularly intended for piston rings, that meets therequirements necessary regarding wear resistance, resilience,anti-corrosiveness, hardness, thermal stability and ductility.

Another object is to provide a coated piston ring, which does not sufferfrom the above drawbacks found in the prior art. Other features andadvantages of the present invention will become apparent from thefollowing description of the invention.

The present invention provides a method of applying a coating materialto a piston ring surface, said method comprising the following steps,application of said coating material by a thermal spray process, heattreatment of said coating material at an elevated temperature and for atime effective to at least partially diffuse said coating material intothe underlying surface, and apply an additional coating material layersubject to successive heat treatments of each said coating materiallayer in order to lay down on said piston ring surface a plurality oflayers of same said coating material. In accordance with the inventivemethod a strong bond between the coating material and the piston ringsurface as well as internally strong bonds within the coating isachieved.

By controlling the heating temperature, to which the coating material isexposed when applied on the piston ring the coating material willdevelop necks in contact points (microwelds) between particles in saidcoating and piston ring surface resulting in a strong bond between saidparticles. Depending on the material of the coating and piston ring thecontact point necks are developed by exposing said material to heatingtemperature below or close to the melting point of the coating material.A coating material intended for piston rings typically comprises amatrix material and a reinforcement material. Conveniently, but notexclusively, the elevated heat treatment of the coating material on thepiston ring is in the range of 60 to 80% of the melting temperature ofthe reinforcement material of the coating. The matrix material willaccordingly be exposed to heat near its melting temperature during shortperiods in time causing the formation of said necks in the material. Bythe inventive method there is provided a coating comprising open poresbetween the particles of the coating. Further, according to theinventive method the object of reducing loose particles in the coatingof a piston ring is met.

Preferably, the piston ring is moved in relation to a thermal spraydevice and a heat treatment device while applying said coating materialand heat treatment to said piston ring. By controlling the movement thetime of exposure to heat can be controlled in a desired manner.Preferably, the piston ring is rotated about its axis, in relation to athermal spray device and a heat treatment device, while continuouslyapplying said coating material and heat treatment.

In order to provide the desired elevated heat treatment of said pistonring induction is preferably used. Heat treatment by induction isconsidered cost efficient but also other alternatives known by a personskilled in the art can be used.

Another benefit with the inventive method is that the resulting pistonring coating has an evenly distributed porosity. Preferably, saidresulting piston ring coating has a porosity of between 1 to 15 vol %.The coating material comprising the yielded pores hence has thepossibility to absorb deformations and imperfections caused by inducedstress.

Open pores can also be used as a buffer for lubricating substances toprovide a lubricant effect on the piston ring during use and hencereduce friction. Closed pores however can not easily receive externallyfed lubricant and are hence of less use in piston rings if this isdesired.

Further, each of said coating material layers typically have a thicknessof between 0.005 to 0.10 mm. Preferably the thickness of a layeraccording to the inventive method is around 0.01 mm. The thickness of acoating layer according to the inventive method is conveniently between0.005 to 0.10 mm in order to develop the desired necks in the contactpoints between particles of the coating layer during heat exposure.

In a preferred method according to the invention, the coating materialis of pulverulent type when fed to said thermal spray process.Alternatively said coating material has a wire like form when fed tosaid thermal spray process in order to achieve an efficient andcontrolled manufacturing process.

In accordance with the invention there is also provided a piston ringcoated with a coating material by a thermal spray process, exposed toheat treatment of said coating material at an elevated temperature andfor a time effective to at least partially diffuse said coating materialinto underlying surface, and an additionally applied coating materiallayer subject to successive heat treatments of each said coatingmaterial layer in order to provide on said piston ring surface aplurality of layers of same said coating material.

In accordance with a preferred embodiment of the inventive piston ring astrong bond between the coating material and the piston ring surface aswell as internally strong bonds within the coating is achieved. Bycontrolling the heat, to which the coating material is exposed whenapplied on the piston ring the coating material will develop necks incontact points (microwelds) between particles in said coating and pistonring surface resulting in a strong bond between said particles.Depending on the material of the coating and piston ring the contactpoint necks are developed by exposing said material to heat below themelting point of the material. Conveniently, but not exclusively, theelevated heat treatment of the coating material on the piston ring is inthe range of 60 to 80% of the melting temperature of a reinforcementmaterial of the coating. By the inventive method there is provided acoating comprising open pores between the particles of the coating.

Further, according to the invention the coating material comprises ametallic compound chosen from a group consisting of Cr₂O₃ and Al₂O₃.These compounds and metal alloys have proven to result in piston ringcoatings of high quality. The coating according to a preferredembodiment comprises a mixture partly in the form of metal, partly inthe form of reinforcement. In accordance with a preferred embodiment ofthe invention the coating material is a cermet. Cermet is a group ofcoatings that combine a ceramic and a metal or alloy. A frequently usedexample is Chrome Carbide (the ceramic constituent) in a Nickel/Chromematrix. It is anticipated that a person skilled in the art may use otherceramic compounds, alloys and cermets (not mentioned here) in order toprovide a coating on a piston ring according to the invention.

Further it is provided according to the invention a piston ring, whereineach of said coating material layers typically have a thickness ofbetween 0.005 to 0.10 mm. Preferably the thickness of a layer accordingto the inventive proton ring is around 0.01 mm. The thickness of acoating layer according to the inventive-piston ring is convenientlybetween 0.005 to 0.10 mm in order to develop the desired necks in thecontact points between particles of the coating layer duringmanufacturing.

BRIEF DESCRIPTION OF THE DRAWING

Currently preferred embodiments of the present invention will now bedescribed in more detail with reference to the accompanying drawing.

FIG. 1 is a schematic representation of an arrangement for using themethod of applying a coating material to a piston ring according to thepresent invention.

FIG. 2 is a schematic representation of an example of a resultingcoating using a prior art method.

FIG. 3 is a schematic representation of a part of a piston ringcomprising a coating in accordance with the invention.

FIG. 4 is a schematic representation of an enlarged partial view of FIG.3 in accordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A currently preferred method according to the invention will bedescribed together with a preferred embodiment of a piston ringaccording to the present invention with reference to the accompanyingdrawings.

Referring now to FIG. 1, a piston ring 1 is arranged for being providedwith a coating. The coating manufacturing arrangement further comprisesa thermal spraying device 3 and a heat treatment device 5. Preferably,but not necessarily, a protective cover 2 is provided for coveringsurrounding elements from e.g. spraying. Further, an inductorconstitutes the heat treatment device 5. The inductor 5 can be appliedin a number of ways depending on the size of the piston ring 1. In thepresent embodiment more than one inductor 5 is shown. A cleaned pistonring 1 is arranged to the coating arrangement. The piston ring 1 isrotatably arranged to the coating arrangement. In accordance with apreferred embodiment of the invention a coating material 4 is applied tothe desired parts of the piston ring 1 by means of thermal spraying. Thethermal spraying device 3 conveniently uses e.g. plasma, arc, HVOF orflame spraying technology, which is conisidered well known for a personskilled in the art.

The piston ring 1 is rotated during the spraying and instantly thecoating material can be subject to heat treatment at elevatedtemperature. According to a preferred method the periphery speed of thepiston ring surface to be coated is around 20 m/min when applying thecoating material and heat treatment. Hence, the revolution speed ispreferably adjusted depending on the diameter of the present pistonring. The coating according to a preferred embodiment comprises amixture partly in the form of metal, partly in the form ofreinforcement. The coating material is preferably heated to between 60to 80% of the melting temperature of the reinforcement material in saidcoating during a short period of time. Approximately 20 cycles ofapplying the coating material 4 and heating the coating material 4 willhence be achieved during 1 minute for a piston ring of conventionaldimensions.

Each cycle will result in a new coating layer 24 on the piston ringsurface, which is illustrated in FIG. 3. In FIG. 3 only three layers 24are shown to illustrate the principle, but normally more than 10 layerswill be applied and preferably more than 50. The number of layers 24 ofcourse depends on e.g. the thickness of the layers 24 and the size of hepiston ring 1 etc. For a normal piston ring 1 according to a preferredembodiment of the invention each coating layer 24 has a thickness oftypically 0.01 mm. The total thickness of the applied coating of apiston ring 1 is preferably around 0.8 mm. These figures are howeveronly given by way of example to give an idea of the properties to beused and it is understood by a person skilled in the art that it ispossible to adjust the properties such as the thickness of the layers24, the revolution speed of the piston ring during manufacturing of thecoating as well as the number of layers 24 etc. in accordance withspecific demands of the presents piston ring 1 to be coated.

Further according to the invention the coating material 4 comprises ametallic compound chosen from a group consisting of e.g. Cr₂O₃ andAl₂O₃. These compounds and metal alloys have proven to result in pistonring coatings of high quality. In accordance with a preferred embodimentof the invention the coating material is a cermet. Cermet is a group ofcoatings that combine at least a ceramic and at least a metal or analloy. A frequently used example is Chrome Carbide (the ceramicconstituent) in a Nickel/Chrome matrix. It is anticipated that a personskilled in the art may use other ceramic compounds, alloys and cermets(not mentioned here in detail) in order to provide a coating on a pistonring according to the invention.

In accordance with a preferred embodiment of the inventive piston ring 1a strong bond between the coating material 4 and the piston ring surface22 as well as internally strong bonds within the coating is achieved. Bycontrolling the heat, to which the coating material is exposed whenapplied on the piston ring 1 the coating material 4 will be forced todevelop necks (microwelds) in contact points between particles in saidcoating and piston ring surface resulting in a strong bond between saidparticles. Conveniently, but not exclusively, the elevated heattreatment of the coating material on the piston ring is in the range of60 to,80% of the melting temperature of at least the reinforcementmaterial of said coating material.

This is further illustrated in FIG. 4, where a schematic enlargement ofthe bonding between particles 21 in one layer 24 is shown. By theinventive method it is provided a coating on a piston ring, whichcoating comprises open pores between the particles 21 of the coating.Depending on the material of the coating and piston ring 1 the contactpoint necks 23 are developed by exposing said material to heat below themelting point of the material. The contact point necks 23 providesadditional strength to the coating. The step of exposing the coating toan elevated temperature will hence cause a significantly stronger bondcompared to the mechanical bonding normally achieved by just thermalspraying.

In FIG. 2 the effect of applying excessive heat, which for otherapplications might be beneficial but not for piston ring coatings, to acoating layer or an entire coating of a substrate is shown. Whenapplying excessive heat during coating of a substrate the coatingmaterial will melt. In order to be able to compare the size of theillustrated coated part in FIG. 2 it has a size that is about equal tothat of FIG. 4. It can be seen that the coating in FIG. 2 show asignificant decrease in porosity, the porosity is not evenlydistributed, and that the pores are closed.

Closed pores are not generally able to provide any lubricant effect.Furthermore, the ductility provided by such a coating with few andunevenly distributed pores is generally not sufficient for use in apiston ring coating. Though the bond will be strong in such a coatingother characteristics, as mentioned, are not adapted to the meet theconditions that will be applied on a piston rings. It has been foundthat, due to the formation of necks within and between the multilayersof the coating during heat treatment according to the invention, astrong bonded coating with excellent ductile properties and wearresistance can be achieved.

Further, there is a risk for other properties of the piston ring to benegatively effected when excessive heat is used applying a coating.According to the invention there is thus provided a method that providesa coating to a piston ring with limited damage to the piston ring blank,good adherence thereto, open pores and a suficient porosity.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent for one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof. Thus it is understood thatvarious methods such as plasma, HVOF, and are spraying or other relatedmethods can be used to apply the inventive composition to a piston ring.The arrangement and method for applying the coating and heat to thecoating on the piston ring 1 may of course be adjusted to present needsand is not limited by the above-described method, which is only given byway of example.

1. A method of applying a wear resistant coating material to a surface(22) of a piston ring (1), said method comprising the following steps,application of said coating material by a thermal spray process, heattreatment of said coating material at an elevated temperature and for atime effective to at least partially diffuse said coating material intothe underlying surface, by exposing said material to heating temperaturebelow the melting point of the coating material, and apply an additionalcoating material layer (24) subject to successive heat treatments ofeach said coating material layer (24) in order to lay down on saidpiston ring surface (22) a plurality of layers (24) of same said coatingmaterial, wherein said resulting piston ring coating has a porosity ofbetween 1 to 15 vol %.
 2. A method according to claim 1, wherein saidpiston ring (1) is moved relatively to a thermal spray device (3) and aheat treatment device (5) while applying said coating material (4) andheat treatment to said piston ring (1).
 3. A method according to claim1, wherein said piston ring (1) is rotated about its axis, in relationto a thermal spray device (3) and a heat treatment device (5), whilecontinuously applying said coating material (4) and heat treatment.
 4. Amethod according to claim 1, wherein said heat treatment of said pistonring (1) is provided by induction.
 5. A method according to claim 1,wherein said resulting piston ring coating has an evenly distributedporosity.
 6. (Canceled).
 7. A method according to claim 1, wherein saidresulting piston ring coating comprises open pores (23).
 8. A methodaccording to claim 1, wherein each of said coating material layer (24)typically has a thickness of between 0.005 to 0.4 mm.
 9. A methodaccording to claim 1, wherein said coating material is of pulverulenttype when fed to said thermal spray process.
 10. A method according toclaim 1, wherein said coating material has a wire-like form when fed tosaid thermal spray process.
 11. A method according to claim 1, whereinsaid heat treatment result results in necks (23) in contact pointsbetween particles (21) in at least said coating.
 12. A method accordingto claim 1, wherein said coating material comprises a metallic compoundchosen from a group consisting of Cr₂O₃ and Al₂O₃.
 13. A methodaccording to claim 1, wherein said coating material is a cermet.
 14. Apiston ring (1) coated with a wear resistant coating material, by athermal spray process, wherein said wear resistant coating has beenexposed to heat treatment of said coating material at an elevatedheating temperature below the melting point of the coating material andfor a time effective to at least partially diffuse said coating materialinto underlying surface, and an additionally applied coating materiallayer (24) subject to successive heat treatments of each said coatingmaterial layer in order to provide on said piston ring surface (22) aplurality of layers (24) of same said coating material and wherein saidpiston ring comprising necks (23) in contact points between particles(21) in at least said wear resistant coating, wherein said piston ringcoating has porosity between 1 to 15 vol %.
 15. A piston ring (1)according to claim 14, wherein said piston ring (1) is moved in relationto a thermal spray device (3) and a heat treatment device (5) whileapplying said coating material (4) and heat treatment to said pistonring (1).
 16. A piston ring (1) according to claim 14, wherein saidpiston ring (1) is rotated about its axis while continuously applyingsaid coating material and heat treatment.
 17. A piston ring (1)according to claim 14, wherein said heat treatment of said piston ringis provided by induction.
 18. A piston ring (1) according to claim 14,wherein said piston ring coating has an evenly distributed porosity. 19.(Canceled).
 20. A piston ring (1) according to claim 14, wherein saidpiston ring coating comprises open pores (23).
 21. A piston ring (1)according to claim 14, wherein each of said coating material layers (24)typically have a thickness of between 0.005 to 0.4 mm.
 22. A piston ring(1) according to claim 14, wherein said coating material is ofpulverulent type when fed to said thermal spray process.
 23. A pistonring (1) according to claim 14, wherein said coating material has a wirelike form when fed to said thermal spray process.
 24. A piston ring (1)according to claim 14, wherein said coating material comprises ametallic compound chosen from a group comprising of Cr₂O₃ and Al₂O₃. 25.A piston ring (1) according to claim 14, wherein said coating materialis a cermet.
 26. A method according to claim 2, wherein said piston ring(1) is rotated about its axis, in relation to a thermal spray device (3)and a heat treatment device (5), while continuously applying saidcoating material (4) and heat treatment.
 27. A method according to claim2, wherein said heat treatment of said piston ring (1) is provided byinduction.
 28. A method according to claim 3, wherein said heattreatment of said piston ring (1) is provided by induction.
 29. A pistonring (1) according to claim 15, wherein said piston ring (1) is rotatedabout its axis while continuously applying said coating material andheat treatment.